It is known that phototrophic microorganisms are already today found in many commercial applications. Thus, algae are produced to manufacture β-carotene, astaxanthin, etc., or the complete algae biomass is sold as nutritional supplement. Today, the production of algae biomass faces two main difficulties. First, a large part of the current production results from open systems (e.g., so-called open ponds). These open systems are sensitive for contaminations by other algae strains or by pest, therefore only algae with very specific growth requirements can be grown in these systems. Thus, for instance the algae Dunaliella is cultured for the production of β-carotene under very saline conditions, which are not suited for most other organisms. Second, the production costs of algae biomass is rather high (>USD 2,000 per metric ton), so that a commercial production for many applications, especially in the energy sector or the transportation sector, is not profitable. In particular, production costs are often increasing even more if closed systems are used instead of open systems to avoid contaminations. Besides the open ponds, a large number of various photobioreactor types are presently in use. Tube reactors, which can consist of one or more horizontal tubes, or wherein a tube is helically wound around a cylinder or cone (biocoil), are among the best known. Furthermore, flat panel reactors are often used, such reactors providing a vertical liquid layer for cultivation of algae.
The main challenges in the production of chemicals and energy from algae are the risk of contamination and the high cost for the manufacturing of the algae biomass. Likewise, the main challenges in the production of fine chemicals, nutritional supplements, vitamins, omega-3-fatty acids, antioxidants (e.g. carotenoids), pharmaceutically active substances or dried biomass for nutritional supplementation from algae are thus the risk of contamination and the high cost for the manufacturing of the biomass. The same challenges apply when culturing algae for biofuels, animal feed, amino acids, methane production, etc.
WO 2005/121309 discloses a device for algae production, in which a fluid comprising algae is located in growth containers, to which a gas containing CO2 is supplied, the gas being circulated through the containers via a gas-conditioning apparatus. In an embodiment the containers are made of a double plastic sheet forming a sheet bag. In an embodiment said to be suitable for the production of micro-algae in the sea, a welded-together sheet bag is placed to float horizontally in the water surface.
U.S. Pat. No. 4,868,123 discloses an apparatus for production of microorganisms by photosynthesis. The apparatus comprises a bioreactor to be placed on an expanse of water, which has a first group of flexible tubes, which are transparent to light and in which the culture medium circulated and a second group of inflatable tubes, disposed and maintained beneath the first group by means of detachable Y-shaped interpolated members, which are regularly spaced. According to this document, when the temperature of the culture medium exceeds an upper reference temperature the photobioreactor is immersed through deflation of the tubes of the second group. Conversely, when the temperature of the medium is below a minimum reference temperature, the tubes of the second group are inflated with compressed air. The immersion of the photobioreactor can also be ensured by introducing a relatively heavy liquid into the tubes of the second group, whilst floating can be ensured by injecting a light fluid other than air.
Since large-scale cultivation of microorganisms is very cost sensitive, there is a demand for simpler and cheaper bioreactor arrangements.